Indirect speech (2)

Please read:

Direct speech:

I must go now. (1)

Case 1: "Must" expresses certainty.

So, indirect speech for (1):

1. She said she must go then.

Case 2: Must expresses necessity.

So, indirect speech for (1) :

1. She said she must go now.

Am I right?If not, please give me examples for these 2 cases with "must" and tell me the rule.

Quoc